The developing field of microfluidics deals with the manipulation and measurement of exceedingly small liquid volumes—currently down to the nanoliter, or even picoliter range. For example, modern analytical integrated circuits (ICs), such as “system on a chip” (SOC) or “lab on a chip” (LOC) biochips, can analyze solutions that are deposited directly on the chip surface. Typically, the surface of an analytical IC will include an array of analysis locations so that multiple analyses can be performed simultaneously. A “microarraying” system that includes a biochip is extremely useful for genetics research because of the substantial improvements in efficiency provided by this parallel analysis capability. To maximize the number of analysis locations (and therefore the number of analyses that can be performed at one time), the size of each analysis location on the biochip is minimized. To ensure that the biological solution from one analysis location does not flow into a different analysis location, an array (often referred to as a “microarray”) of microfluidic “printing tips” is used to dispense a precise volume of the biological solution at each analysis location.
FIG. 1 is a perspective view of a conventional microarraying system 100, which includes a stage 110 for supporting a biochip 120, a microarray 130 mounted to an XYZ positioning subsystem 160, and a computer/workstation 170 that serves as both a system controller and a measurement data processor. Microarray 130 includes a plurality of printing tips 150 mounted in an array formation on a mounting base 140. XYZ positioning subsystem 160 moves microarray 130 in response to control signals provided by computer/workstation 170 to collect and dispense samples of test solutions in an array pattern on biochip 120. A channel 151 cut into the end of each printing tip 150 stores and dispenses tiny samples of the test solutions onto specific analysis locations on biochip 120. In this manner, each printing tip 150 acts as a microfluidic conduit—i.e., a transport pathway for microfluidic volumes of liquid. Biochip 120 then analyzes the samples in parallel and provides the results to computer/workstation 170 for further processing. As mentioned previously, this type of concurrent analysis greatly reduces the amount of time required to perform a set of experiments.
To ensure that test samples are accurately and evenly placed on biochip 120, printing tips 150 in microarray 130 must be made to extremely tight tolerances and must be precisely arranged in microarray 130. As the number of pins is increased to allow larger numbers of samples to be concurrently tested, the dimensional requirements only become stricter. As a result, the microarrays used in conventional microarraying systems are expensive and difficult to manufacture. For example, companies such as Oxford Lasers, Ltd. manufacture the metal pins used to dispense biological solutions in microarraying systems such as microarraying system 100 using techniques such as electro-discharge machining (EDM) and copper-vapor-laser (CVL) micro-machining. The minimum channel width in such pins is roughly 10 μm, with each pin being fabricated individually and taking up to 30 minutes to complete. Once pin formation is complete, the pins still must be assembled into the high-precision microarray, which adds additional time and expense to the manufacturing process. This low production throughput (Oxford Lasers, Ltd. is currently manufacturing about 1000 pins/months for BioRobotics) means that the final microarrays are extremely expensive. This in turn impacts testing throughput, since the high cost of the microarrays mandates that they be reused rather than replaced. Therefore, to prevent cross-contamination, the microarrays must be meticulously cleaned, which can be very time-consuming.
Even for microfluidic systems using a smaller number of printing tips, pin costs can be problematic. For example, FIG. 2 shows a perspective view of a dip pen nanolithography (DPN) system 200. DPN system 200 includes a stage 210 for supporting a wafer 220, a micropen assembly 230 mounted to an XYZ positioning subsystem 260, and a computer/workstation 270 that serves as a system controller. Micropen assembly 230 includes a printing tip 250 mounted in a mounting base 240. XYZ positioning subsystem 260 moves micropen assembly 230 in response to control signals provided by computer/workstation 270 to print a desired pattern on wafer 220. A channel 251 is cut into printing tip 250 to allow the printing tip to act as a microfluidic conduit and apply a print solution onto wafer 220. This type of micropen-based lithography can allow more complex and detailed patterns to be printed than would be possible using conventional lithography techniques. However, as with microarraying systems, the difficulties in fabrication and the high cost associated with the metal pins used in micropen assemblies limit the usefulness of current DPN systems, for much the same reasons as were previously described with respect to conventional microarraying systems. Alternative DPN systems, such as the lithographically-formed planar beams with perpendicular printing tips is described in “A MEMS Nanoplotter with High-Density Parallel Dip-Pen Nanolithography Probe Arrays”, Zhang et al., Nanotechnology, v13 (2002), pp. 212-217, present other difficulties, as the flat configuration of the planar beams can consume significant die area, thereby limiting the printing tip density, and the printing tips themselves require delicate sharpening operations that can adversely impact both yield and cost.
What is needed is a microfluidic conduit that can be produced and formed into microfluidic devices such as microarrays and micropen assemblies without the manufacturing difficulties and high costs associated with conventional metal pins.